Arrangement of carrier channels in multiplex signaling systems



April 10, 1928.

w. B. NOTTINGHAM ARRANGEMENT 0F CARRIER CHANNELS IN MULTIPLEX SIGNALING SYSTEMS Filed Dec. 24, 1925 )mv 1 W J m W1 I n w fi mm d A ,m M. fi mm 1 n 4% EN m, W0: A z a p M w 5 F M 1 M. .4 5% Mm 0 F P P9,. H W 0 6w mm w o e I 8 1 M I .w .i 7 F 7, L fl F a z e i g .m 19/. M E? m 4 (M v 6 w 5% 0 :2 a z r 6 gm n U Ee y m a, E m, m W m r A F .M r c Z w M M 0 S l flgp m :8 J 1 2. a m mm L l .14 b w 2 0.:v m fi fiu vztu Patented Apr. 10, 1928.

UNITED STATES WAYNE B. NOTTINGHAM, OI PHILADELPHIA, PENNSYLVANIA, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, NEW YORK.

01? NEW YORK, N. Y., A CORPORATION OF ARRANGEMENT OF CARRIER CHANNELS IN MULTIPLEX SIGNALING SYSTEMS.

Application filed December This invention relates to multiplex carrier signaling systems, and particularly to arrangements of the carrier channels in such systems.

An object ofthe invention is to reducecross-talk interference between the channels of multiplex carrier signaling systems.

A feature of the invention is a multiplex carrier signaling system in which the necessary amount of attenuation of the high frequency-leakage between the paired receiving and transmitting channels is materially reduced.

In accordance with the invention, in a multiplex carrier signaling system inwhich all of the channels transmitting in one direction employ carrier frequencies below a.

certain value, and all of the channels transmitting in the opposite direction employ carrier frequencies above thisvalue, the oneway channels utilizing the lower-group of frequencies are paired with the one-way channels utilizing the higher grpup of frequencies to form the opposite sides of twoway channels in such a manner as to make the nearend interferencebetween the channelllsI appear as side tone rather than crossta Side tone may be defined as the measure of .the interference between the transmitting and receiving circuits at the same ter minal of the same two-way channel, while near end cross-talk as used in this'specification may be defined as the interference produced in the receiving circuit of one channel by the transmitting circuits of one or moreother channels at the same terminal of the multi-channel system. The objects and features of the invention -will be clear from the following detailed description when read in connection-with the v accompanying drawing, inwhich the single figure shows a terminal station of a multiplex carrier telephone system embodying the invention and curves illustrating the principles of the invention.

. The invention will ,be described in connection with a three-channel multiplexcarrier telephone system of the carrier suppression type, although it is applicable as well to other types of multiplex car-rier systems:

In such a system, the'carrier wave of each channel is suppresed from transmission when no signals are bemg sent, and when 24, 1925. Serial No. 77,478.

signals are being transmitted, modulation components of the modulated wave are sent over the line, but no unmodulated components' of the carrier wave are transmitted. In order to reproduce, the signal from modu lated waves ofthis character, it is necessary that the carrier wave .supplied to the demodulator at the receiving station be of substantially the same frequency as the carrier wave which was suppressed at the transmitting station. A system of this general character is described in connection with Fig. 49 of an article entitled Carrier current telephony and telegraphy by Messrs. Colpitts and Blackwell published in the Transactions of the American. Institute-of Electrical Engineers, volume 40, 1921.

The single figure of the drawin shows the east terminal of a three-channe carrier telephone system ofthe-t pe described above. As the west terminal not shown) of the system has terminal apparatus identical with.

and arranged in the same manner as that at the east station, only the east terminal need be described.

The terminalapparatus shown in Fi 1 comprises three transmitting channels T0 TG,, and TC connected through a common transmitting circuit TL to the multi lex I line ML, and three receiving channels 0,, RC and RC, connected through a common receiving circuit RL to the multiplex line ML.

"The carrier currents utilized for transmission over the line ML are grouped as to their frequencies, the higher frequencies as a group, being used for transmission from west to east and the lower frequencies as a group being used, for transmission from east to west.

Included in the common transmission circuit TL connecting the transmitting channels T0,, T0,, and TC to'the multiplex line ML are a transmitting am lifier TA and a low-pass grouping filter L F, and in the common receiving circuit RL cohnecgig the receiving channels RC RC, and to the multiplex 'line areceiving amplifier' RA and. a high-pass grou ing filter HGF. -The filters HGF andLGl serve to separate the'directional groups of carrier waves to they respective terminal transmitting and receivingcircuits. These filters and each of the other filters shown thrcughof signaling lines, are balanced with artificial networks N,, N and N and are associated with the respective transmitting and receiving circuits through the differential repeating coils H H and H commonly known as hybrid coils, which enable independent transmission in the two directions etween the lines and high frequency terminal apparatus.

Transmittin channel TC includes a modulator M anifa band filter TBF,, transmitting channel T a modulator M and a band filterTBF and transmitting channel TG a modulator M and a band filter TBF,. Receiving channel RC, includes a demodulator DM, and a band filter RBF,,

receiving channel RC a demodulator DM y of the freqi lency as'si I modulated and a band filter RBF,, and receiving channel RC a demodulator DM and a band filter RBF balanced type disclosed in the U. S. patent toCarson 1,343,306, June 15, 1920, desi ed to suppress the unmodulated carrier rom transmlssion. The demodulators DM DM and DM, may be of the well-known vacuum tube balanced t pe shown in Fig. 49 of the C01 itts and Blhckwell article, supra.

- Carrier currents of the frequency assigned to channel TC, are su TO to the modulator 1 wherein they are modulated 'by 'voice frequency currents or other si aling currents from the low fre quency hne L Similarly carrier frequencies ed to the transmitting channels. C and C, are supplied from sources TO, and T0,. respectlvely' to the modulators M and M whereim they are p 'by' signaling currents from the low fr Similar y, carrier sources R R0,, and R0, are associated with the demodulators DM,, D11 and DM to supply the incom side bands in each receiving circuit with the carrier current which was sup pressed at the other end of the line ML. The low-pass'filters F F and F," are respectively inserted in the connections be tween re iving channels BC BC and 3 and the particular transmitting channels with whic they are aired, and are used to firsvent the carrier in the and R0 from leaking transmitting channels through unbalance in 'the hybrid coils.

The band filters TBF,, T F,, and-TBF in transmitting channels TC T0,, and T0,, val and the band filters RBF F amfBBF, inthe receiving channels the. principles set -forthin the.

, receiving amplifier RA, The modulators M M and M are of the plied from .a. source .points as to divide the available range, which. in practice isabout 3 to uency lines L and L6 respectively.

ting west and the up r' to 30 kilocycles, isitofi ufreq east. The width of the side 'tively, and to RC and R0,, 16,100, 19,750, and 23.400

RC RC and RC respectively, are ;designed so as to transmit one of the s1de bands, either the 11 per or lower as desired,

produced by modu ation in the respective channels and to'suppress from transmission frequencies lying outside of such bands.

Voice frequency currents ori inatlng, for

*TBF, suppresses all exceptone side band,

for example, the lower side band which it transmits to the common transmission cirinstance, in the low frequency line L at the cuit TL. This side band is amplified by the amplifier TA and then is pass grouping filter LG to the multiplex line ML and thence to the west station.

Similarly, the incoming side band received over the line ML from the west station is assed by the'lowpassedby the high-pass firoupin filter HGF- to the receiving circuit L, amp ified by the and selected bythe respective receiving band filters in the receiving channels for transmission to the respective low frequency lines. For example, currents passing through the receiving band filter RBF are combined'in the modulator DM with .carrierwaves from the source of dmodulation in the output circuit of the modulator 'DM, are passed through the lowpass filter L and through the particular hybrid coil connected thereto to one of the low frequency lines. In the system as shown in the fi re, the low-pass grouping filter LGF and t e highpass grouping filter HGF have such 'cutflofi carrier k1locycles,--into two parts at a frequency of'about 15.0 kiloc'ycles. As stated previouslg it is assumed that the lower part of the 'equency range, that is, 3 to 15 kilocy is to be utilized for the channels transmituency range, 15 for channels transmitting bands transmitted by each of the channels and the voice frequencyacomponents cles,

is. selected from quality considerations as 1 2500 cycles.

It will be assumed that the carrier frequencies assi ed to the transmitting channels T0,, are 14,000, 10,700, Iand 7 .700 c cles, res

the receiving 0 annels cycles, respectively. g It Wlll also be assumed and T0 at the west station that the transmitting band filters in the sy's-- tem are such as to transmit the lower side bands and the receiving band filters such as to transmit the upper side bands) For pracline tieal considerations, in such a system it is advisable to set the carrier frequency. in each channel at some point slightly outside the transmitted bands of the band filters. The transmission ranges of the transmitting band filters TBF TBF and TBF will accordingly be selected as 11,30013,800 cycles, 8,00010,500 cycles and 5,0007,500 cycles, respectively, and those of the receiving-band filters REF v RBF,, and RBF 16,300 18,800 cycles, 19,950-22A50 cycles, and 23,60026,100 cycles, respectively.

The free transmission bands of the respective transmitting and receiving band filters, and of the grouping filters of the system and their cut-off points are indicated graphically in the diagram. associated with the figure. In this graphical diagram, frequency is plotted as abscissae and attenuation as ordinates. The particular curves corresponding to the dilferent filters are indicated bycharacters similar to those used for designating-the filters in the system. The direction oftransmission and the position of the carriers with reference to the transmission bands of the filters are indicated by the vertical, full-line-arrows. y

The particular division of the available carrier range described above and illustrated in the figure is one that is being used in present installations of three-channel carrier telephone systems, and has been selected in accordance with practical considerations.

The ranges of the respective band filters in the transmitting and receiving channels of the system having been fixed, the problem is to pair the one-way channels to form the opposite sides of the two-way channels in such a way as to reduce cross-talk interference due to interinodulation between the side band frequencies in the respective channels to a minimum.

This has been accomplished in accordance with the invention by pairing a channel utilizing an intermediate carrier frequency in the lower group with the channel utilizing the lowest carrier frequency in the upper group, and airing the channel utilizing the highest carrier frequency in .the lower group with a channel utilizing an intermediate carrier frequency in the upper group. Accordingly as shown in the figure low frequency L is associated through hybrid coil H with transmitting channel TC, and with receiving channel RC and low frequency line L is associated'through hybrid coil H with transmitting channel TC and with receiving channel RC The low frequency line L is associated through hybrid coil H with transmitting channel TC and with receiving channel RC The advantage of such an arrangement will be brought out in the following discussion. e

, Let it be assumed that the channels TC,- TC,, and TC, are being simultaneously used for transmitting messages. Then each side band frequency transmitted by any one of the transmitting band filters may combine in the transmitting amplifier TA with all of the side band frequencies transmitted by the other transmitting band filters to produce modulation products of second, third, and higher. orders, which products pass through the grouping modulation products are of such a level that even with the loss introduced in them within the suppression-range of the low-pass grouping filter LGF, they are of the degree of magnitude of the low level currents coming over the line, and consequently are passed through grouping filter HGF in receiving circuit RL with the incoming line currents, amplified by the receiving amplifier RA, and are effective in the individual receiving channels. Of these modulation components, only thethird order products need be considered as they are the only troublesome products from the standpoint of cross-talk.

If s s and s, are side band frequencies passed by the transmitting band filters TBF,, .TBF and TBF respectively, the third order modulation products resulting are: I

The pairing of the transmitting channels with the receiving channels as described above and illustrat d in the figure, makes the particular ones of the above third order modulation productswhich are objectionable appear 'in the receiving channels as side tone rather than as cross-talk as in the case of other arrangements, This may be shown as follows:

It will be assumed, first, that the carrier channels, instead of being paired in the manner shown in the drawing, are paired, as in the usual manner, symmetrically with respect to the frequency spectrum, between the two groups utilized for transmitting in opposte directions, that is, the 14,000 and 16,100 carriers'are directed channelsof one circuit, the 10,700 and 19,750 cycle channels of a second circuit, and the 7,700 and 23,400 cycle channels of a third circuit. With such an arrangement low frequency line L would be assoc ated through hvbrid coil- H, with transmitting channelTO and receiving channel 110.. low frequency line L, through hybrid coil with transmitting channel TC and receiving channel RC.,, and low frequency line L}, through hybrid coil IL. with transm tting channel TC and receiving channel RC With such an arrangement, voice frequenciesltransmitted by transmitting band filter LGF. Some of these paired as the oppositely" filterfTBF in channel TC and voicefrequencies transmitted by transmitting band 7 filter TBF in transmitting channel TC will combine in amplifier TA to produce third order modulation products which will betransmitted to and fall within the range of the receiving band filter RBF in receiving channel RC and cause objectionable interference with message currents transmitted thereto over the line MD from its corresponding transmitting channel at the-west end of the line. An example of this is the following:

The particular frequency having thev greatest amplitude in the side band transv mitted by transmitting band filter TB]? is 1000 cycles below the carrier or 13,000 cycles. For purposes of illustration consider a frequency 2000 cycles below thecarrie-r' in-the' side band transmitted. b the band filter TBF or 5700 cycles. I we'considerthat channels TC and TC are transmittingmessages simultaneousl then the side band frequency, 13,000 eyc es, will combine in the- --tran'smitting amplifier TA with the side band frequency, 5700 cycles, and form as one of the third order modulation products,

28,-;8 or 2X13,00057 00=20,300 cycles.

, modulation product selected for illustration is the most The particular third order objectionable. This frequency of 20,300 cycles is transmitted through low-pass grouping filter LGF and high-pass grouping filter HGF, is amplified by the receiving amplifier RA, and as it is within the transmissionrange, 19,950-22,450 cycles, of receiving band filter RBF in receiving channel RC it will' be transmitted thereby and cause intertference with the message currents re celved in that channel. As receiving channel RC is paired with transmitting chan-' nel TC and not with 'the transmitting channels T0 and TC producingthe ob ection- .able third order termodulation product will appear in re-' ceiving channel RC as cross-talk. In othermodulation product, the in words, when two of the channels are being used-for transmitting messages, there will be such objectionable crosstalk in the receiving apparatus connectedwiththe third channel as to interfere with regular :reeeption in that channel.

We will now considei ithe same example in connection with the pairing of the channels in accordance with the invention, that is, where receiving channel RC is paired with, transmitting channel T0,, receiving channelRC, paired with .tiransmittingchanzit nel T0 and' receiving channel RC paired with transm t tingchannel TC as shown in the figure, As in, the previous case the voice frequency of v13,000 cycles transmitted by transmitting band filter lTBF' in transmittingchannel TC will combine in the transmitting amplifier TA with the voice fre- RC eeann quency of 5,700 cycles transmitted by transmitting band filter TBF in transmitting channel and form as one of the third order modulation products, 28 's =20,300 cycles, which will be transmitted through low-pass grou ing filter LGF and high-pass group ing lter HGF, amplified by receiving am-v plifier RA, and-fall within the range of the receiving band filter RBF in receiving channel RC As receiving channel RC in this case is nel TC asin the. previous arrangement, the inter-modulation. product-20,300 cycles will appear in receivingchannelRC, as side tone rather than as; cross-talk.;=j' In nther words with. the arrangementofthe invention, when two of the; channels are being" used';fi'wr transmitting l'rness'agesin one direction -the '-frequency "components due to intermodulation of the voice frequencies transmitted by their respective transmitting band ,filters infthe transmitting amplifier will only cause interference in the receiving apparatus paired with one ofsaid channels, andwill not interfere with'the reception in the third ()hllD-' nel. y

The arrangement of the carrier'channels in accordancewith the invention has the ad ditional advantage over other arrangements in that the attenuation requirements of the low-pass filtersiwhich must be inserted be-' paired with transmitting chan-. nel T0,, instead of with transmitting chan-' nelTC recelving' channeYRC with transmitting channel TC and receiving channel BC, with transmitting channel TO asin the usual manner. 19,750 cycles through the hyhridcoil H into the m0dilla tor M of'transmitting channel TC and combine thlerein with the carrier-frequency 10.700 1 l5 prmlncts,-

cycles, assigned to that channel .toproduce the second order-(modulation- 4 19,750+ 10,'z00=,30,450 yclesand 19,750-

Then the carrier frequency associated with the demodul'a tor DM in receiving channel RC willgleakz' 10,700=9,050 cycies'of whichthe 9,050 cycle component falls within the range, 8,000- 10,500 cycles, of transmitting band filter TBF In order to prevent this inter-modulation product from being objectionable, the low-pass filter-F inserted betweenthe demodulator D M and modlulator 'M, must havealarge attenuation requirements.

in accordance with the invention, receiving channel 11C mjt-ting channel TC receiving channel that is,

Now,-- suppose;the channels are arranged paired. with transmitting channel =TC paired with" trans- 19.750 14,000 5,7 50 cycles and 19,750

14900 33350 cycles, both of which are outside the range of transmitting band filter TBF inthat channel. Also the high frequency carrier 16,100 associated with the demodulator DM, in channel RC, will leak through hybrid coil H into the modulator M in transmitting channel TC and combine with the high frequency carrier 10,700 cycles assigned to that channel to produce the second order modulation products 16,l0010,700=5,400 cycles and 16,100+ 10,700=26,800 cycles, neither of which are within the transmission range, 8,000 1 0,700,

of transmitting band filter TBF in that channel.

It is evident, then, that with the pairing of the channels as in the invention, that the attenuation requirements of the low-pass filters inserted between transmitting and receiving channels to attenuate high frequency leakage therebetween are much less than that required with other arrangements, thereby reducing the cost of such filters.

In the foregoing discussion reference has been made to certain definite frequencies for the purpose. of giving concrete examples. It is to be understood, however, that the principles of the invention are independent of the particular frequencies and frequency ranges hereinbefore referred to.

The invention has been descri ed in connection with its application in a t rec-channel multiplex telephone system of the carrier suppression type, but it is to be understood that in accordance with the spirit of the invention as defined in the appended claims it is applicable as well to other types of multiplex carrier systems having any number of channels.

What is claimed is:

1. In a multiplex carrier wave signaling system comprising a plurality of two-way carrier channels employing different carrier frequencies, all said channels used for transmission in one direction being comprised in a range below a certain limiting frequency, and all of the carrier frequencies in said channels used for transmission in the opposite direction being comprised in a. range above said limiting frequency, the method of reducing near-end cross-talk which consists in selecting the particular carrier frequencies associated in each two-way channel for transmitting in of the carrier frequencies in opposite directions so that objectionable frequency components produced by intermodulation between the frequency bands transmitted in the same direction in any two of said channels will'lie outside the range of the frequency bands transmitted in. the opposite direction in any of the other channels.

-2. In a multiplex carrier wave signaling system comprising a plurality of two-way channels distinguished from one another by the carrier frequencies they employ, the carrier frequencies being included in two separate groups, those used for transmission in one direction being all higher than a definite group-separation frequency, and those used for transmission in the opposite direction being all lower than said group-separation frequency, the method of reducing near-end cross-talk which consists in selecting the particular carrier frequencies to be paired in each of said two-way-channels for transmission in opposite directions'so that objectionable frequency components produced by intermodulation between the side band frequencies transmitted in the same direction in any two of said channels will fall within the range of the side band transmitted in the opposite direction in one of said two channels and outside the range of the side bands transmitted in the opposite direction in the others of said two-way channels.

3. In a multiplex carrier signaling system, a plurality of two-way transmission channels, each of said channels utilizing a different carrier frequency for transmission,

said carrier frequencies used for transmit.-

ting in one direction being all higher than a certain definite fre uency, and those used for transmitting in t e opposite direction all lower than said definite frequency, the oneway channels that are paired with the res ective one-way channels throughout one directional group in order, being distributed throughout the other directional group in a different respective order such that the nearend interference between the channels will appear as side-tone rather than cross-talk.

4. In a carrier signaling system, a plurality of one-way carrier 0 annels, each employlng a different carrier frequency, the carrier frequencies of all of said channels transmitting in one direction being below a definite limiting' frequency, and the carrierfrequencies of all of said channels transmit ting 1n the opposite direction being above said limiting frequency, the one-way channels utilizing the lower group of frequencies frequency such that the objectionable modulation products produced by a combination of frequencies in 'two of said channels will be received in one of said two channels rather than in a third channe 5; In a multiplex carrier wave signaling system, .aplurality of two-way transmission c annels, each employing a different carrier frequency, the carrier frequencies of the channels used for transmitting in one direction being comprised in an u per range, and the carrier frequencies use for transmitting in the opposite direction being comprisedin a lower range, the pairing'of the oppositely directed carrier frequencies of the respective ranges to form the opposite sides of said two-way channels being such that the uppermost carrier frequency of the lower range is paired with an intermediate carrier frequency of the up er range, and the lowermost frequency of tlie upper range is paired with an intermediate carrier frequency of the lower range.

6. In a multiplex carrier wave signaling. -s stem, a plurality of two-way transmission c annels, each utilizing a different carrier frequency, all of the said channels used for transmitting in one direction employing carrier frequencies com rised in a range below a certain value, an all of said channels used for transmitting in the opposite direction employing carrier frequencies comprised in a range above said value, the pairmg of the oppositely directed carrier freqrliency waves in the different ranges to form range is paired with an intermediate carrier frequency of theupper range.

7. In a multiplex carrier wave system employing waves of different frequency for the different simultaneous transmissions in which intermodulation components between channels transmitting in the same direction fall within the frequency range of certainof the oppositely directed channels, means permanently associating'together as the two sides ofaconversational circuit a said one of said certain channels in which the intermodulation components appear and one of the oppo- WAYNE B. NOTTINGHAM,

sitelv directed channels whichgive rise to 

